The present invention relates to an apparatus for indirect heat transfer between two liquids, and more particularly, to single shell boilers used for heat transfer between first and second isolated tube bundles located within the shell using a liquid heat transfer medium.
Heat exchangers for indirect heat transfer between two liquids are generally known, for example as shown in U.S. Pat. No. 2,119,091. Such heat exchangers are typically located within a closed vessel and include a first bundle of tubes for carrying a liquid which acts as a heat source and a second bundle of heat receiving tubes for carrying a liquid which is to be heated indirectly from the heat source via a heat transfer medium located within the vessel.
One known heat exchanger, shown in cross-section in FIG. 1, provides a heat source tube bundle 2 at the bottom of a closed vessel 3 which is surrounded by the liquid heat transfer medium. Heat is transferred from the heat source tubes 2 to the liquid heat transfer medium such that the liquid heat transfer medium boils, releasing vaporized heat transfer medium from the surface. The vaporized heat transfer medium is then carried by thermal convection to the second bundle of tubes 4. The vaporized heat transfer medium condenses on the second bundle of tubes 4 transferring heat through the second bundle of tubes 4 to the liquid medium carried by the second bundle of tubes 4. Condensate droplets of the heat transfer medium then drip from the second bundle of tubes 4 back to the pool of liquid heat transfer medium surrounding the first bundle of heat source tubes 2.
Typically, a layer of froth forms on the pool of liquid heat transfer medium. The froth inhibits the release of vaporized heat transfer medium from the surface. Additionally, the liquid heat transfer medium surrounding the heat source tube bundle is stratified with the hottest, highest pressure liquid heat transfer medium being located at the bottom of the vessel. This also reduces the efficiency of heat transfer to the liquid medium to be heated in the second tube bundle.
It is desirable to provide a means for reducing the layer of froth in the known system in order to improve the release of vaporized heat transfer medium as well as to reduce or eliminate the stratification in the liquid heat transfer medium to improve the heat transfer efficiency.